Sample holder for decomposition or analysis of sample materials

ABSTRACT

In a sample holder (9) for decomposition or analysis of sample material by heating the sample material in a heating appliance (1) such as a microwave oven, comprising a receiving part (11) having a filling opening (15) for the sample material, a lid (14) to close the receiving part (11) and a valve (16) in the upper part of the sample holder (9) that comprises a valve seat and a valve member that bears, counter to its direction of closure, against an elastically yielding thrust piece (31), the cross-section of the thrust piece (31) in the plane running in the direction of closure of the valve (16) is shaped as a plate spring.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a sample holder for decomposition or analysisof a sample material during heating thereof.

BACKGROUND OF THE INVENTION AND PRIOR ART

In a sample holder of this kind one or more materials are simultaneouslysubjected, for the purpose of decomposition or analysis, to chemicaltreatment in which heat may be supplied to promote the reaction Anexample of a sample material to be investigated is sewage sludge that isto be decomposed with highly aggressive acids. To accelerate thedecomposition, i.e. the chemical process, it is usual to heat the samplematerial with the addition of acids or other decomposition agents byheating the sample holder with the sample material and the decompositionagent in a heating appliance, preferably by subjecting it to microwaveradiation in a microwave oven.

The heating can result in a large excess pressure in the sample holder.To prevent damage to or explosion of the sample holder a valve isprovided in an outlet opening that automatically opens if apredetermined internal pressure is exceeded.

In German patent application 38 39 901 a sample holder of theabove-mentioned kind is described that comprises a bottom container partwith a pot-shaped cylindrical lid that can be screwed in an invertedposition on to the pot-shaped cylindrical bottom container part. In thisarrangement the valve forms an integral part of a valve body that can bescrewed on to a connecting piece that is located centrally on the lidand incorporates the outlet opening. The valve is situated in an outletpassage that passes through the valve body and includes a conical valveseat against which a valve member having a corresponding conical headcan be prestressed from above by an amount such that when the internalpressure in the sample holder reaches a predetermined value the valvemember lifts off under this internal pressure and frees the outletopening. The valve member can move vertically in a plug that can bescrewed from above into an open-topped recess in the valve body. Theprestressing of the valve member is done through a thrust piece ofelastic material, preferably silicone, that is located between the valvemember and the plug. The plug can be screwed into the valve body in themanner of a press screw, by means of a turning member at its upper end,until the elastic thrust piece is compressed, thereby producing theprestressing force. The depth to which the plug is screwed in, and thusthe prestressing force, can be determined by means of indicating markson the valve body and/or on the turning member.

This known arrangement has proved to be practicable and useful. However,it is complicated and expensive to manufacture, since a special valvehousing has to be made and fitted. The valve housing seated on the lid,or the valve body, also increases the height of the sample holder.Screwing the additional valve housing or valve body on and off is alsoregarded as a time-consuming operation. Apart from this, screw threadson the top and bottom parts of the container are expensive and makemanipulation slow and difficult. Furthermore thick walls are necessary,leading to long cooling times and increased material requirements andweight. There is also a risk of damage to the sample holder bydistortion and ageing, particularly in the region of the screw threadsand with long decomposition times.

The heating of the sample material in the sample holder is preferablydone in a microwave oven, in which several sample holders can be heatedat the same time. To ensure uniformity of heating a rotary stand isprovided in the oven on which the sample holders can be rotated about avertical axis.

To provide resistance to the corrosive acids that can be generated inthe course of the decomposition and to make microwave heating possible achemically inert material that is transparent to microwaves is required.For this purpose a heat-resistant and chemically inert plastic materialis very suitable.

In an arrangement known from DE-B-36 20 381, having a pot-shapedreceiving part with a lid in the form of a disc resting on its rim, thevalve member is formed by the lid itself and a flat bearing disc ofaluminum is arranged on the lid. To use a sample holder with a valveconstructed in this way for the decomposition of sample materialrequires not only a stand in the oven but also a retaining meansextending over the sample holder to hold the lid in its closed position.For this purpose a base plate is provided beneath the sample holder anda retaining plate that extends over it, the retaining plate having apress screw arranged to be screwed into it from above and coaxially withthe sample holder so that its lower end presses vertically against thebearing disc and thus holds the valve in its closed position. Underexcess internal pressure in the sample holder the lid can distortslightly and rock about the press screw. Oscillatory bending upwards ofthe lid will clearly occur in some position at its periphery. DE-B-36 20381 does not say whether or not the bearing disc participates in thismovement of the lid. It must be assumed that the aluminum bearing discwill likewise bend upwards and outwards under the increased internalpressure at some position on the periphery, so that the lid follows thismovement and the excess pressure can blow off through the resulting gap.The internal pressure will obviously always seek out the weakestperipheral position on the lid and/or the bearing disc. At any rate,what will happen is a very indefinite opening, since both in the case ofone lid in this peripheral region and also when there is more than onelid (simultaneous heating of several sample holders) blowing off willoccur at different internal pressures as a result of differences in thebending strength of the lid or lids. This is undesirable, since atuncontrolled different internal pressures different decompositionreactions can occur and thus falsify the results of the decomposition orthe analysis. With this known arrangement there is also the danger thatthe aluminum bearing disc may be permanently deformed either on screwingup the press screw or by the internal pressure, thus impairing thefunctioning of the sample holder through premature opening or making itunusable. A further disadvantage is that this known arrangement is notsuitable for heating in a microwave oven.

OBJECT OF THE INVENTION

It is an object of the invention to improve the functional stability ofa sample holder of the abovementioned kind.

SUMMARY OF THE INVENTION

In the sample holder of the invention according to one aspect of thepresent invention, the thrust piece is in the form of a plate spring. Asa result of this form the thrust piece is mainly subjected to bendingstresses, and not to compression. This enables substantially uniformelasticity characteristics to be obtained both for one thrust piece of asingle sample holder and also for a plurality of thrust pieces of aplurality of sample holders, so that the stress exerted on the valve bythe thrust piece can be more easily controlled and determined. It isthus simpler to make thrust pieces having substantially uniformelasticity characteristics.

A further advantage of the sample holder of the invention according tothis aspect of the invention is that it makes it simpler to achieve botha small sectional size for the thrust piece and greater spring force,and in addition a greater spring excursion of the thrust piece. Thelatter is important from the point of view of manipulation, namely inthe screwing up of the pressure screw of the retaining means against thelid. When a larger spring excursion is available, any differences inpressure that may occur will have less effect on the opening pressure,since smaller differences in closure forces can be obtained. Largerdifferences in closure forces would lead to there being differentopening pressures, which is undesirable. It is particularly advantageousif the compliance of the thrust piece follows a characteristic curvethat does not increase. Compliance of this kind is comparable with aweight, which always exerts the same force irrespective of the distancetravelled. With an arrangement of this kind the operator does not haveto pay attention to the tension or torque with which the press screw isprestressed against the lid. Even if the press screw were to be screwedin rather more than necessary this would not lead to any increase in theclosure force, owing to the non-rising characteristic curve.Consequently the thrust piece always exerts substantially the sameclosure force irrespective of how much it is pre-stressed by the pressscrew, so that the respective valve will always open at the same openingpressure. Permanent deformation of the thrust piece is avoided and longlife is achieved.

In the sample holder of the invention according to another aspect of theinvention, the valve is stabilised by the presence of the dimensionallystable insert. This gives the manufacturer freedom of choice in respectof the material for the lid, so that he can use a softer and/or lessheat-resistant or chemically inert and less dimensionally stablematerial for it. A particularly suitable material for the lid is a PTFEderivative or Teflon, which is relatively soft particularly at highertemperatures. This is unimportant, since the dimensional stability ofthe insert replaces that of the lid. The dimensional stability is thusmaintained, so that the elasticity of the thrust piece actssubstantially without distortion, which is a further reason why thevalve will always open at a definite, and preferably constant, openingpressure.

The thrust piece preferably consists of a hard or viscoelastic plasticsmaterial, for example HTC (high temperature compound) plastic.

In the sub-claims advantageous features are set forth that contribute tothe achievement of the present objects and give embodiments that aresimpler, cheaper and smaller and also enable different opening pressuresof particular orders of magnitude to be obtained should this be desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of examplewith reference to the accompanying drawings, in which:

FIG. 1 shows a perspective view of a simplified heating appliance with acarrier for sample holders for decomposition or analysis of samplematerial;

FIG. 2 shows on a larger scale a radial vertical partial section throughthe carrier in the region of a position for standing a sample holder;and

FIG. 3 and FIG. 4 show a partial section corresponding to FIG. 2 with amodified form of sample holder.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The heating appliance 1 is a microwave oven having a housing 3 with adoor 4 on its front face, enclosing a heating chamber 2. In the bottompart of the heating chamber 2 is the carrier 5, preferably in the formof a round plate 6, that can rotate about a vertical axis 7. On thecarrier 5, and preferably distributed uniformly around a pitch circle,there are positions 8 for placing or standing sample holders 9, one toeach position In the present embodiment six positions 8 are provided forsix sample holders 9, two of which are shown opposite to one another.

The sample holders 9 are preferably all of the same form, eachconsisting of a pot-shaped housing 11 having a circular horizontalcross-section with a horizontal base 12 and a cylindrical wall 13extending vertically and having a flanged rim 13a. The opening 15 of thehousing, which can be closed by a lid 14, is bounded by the upperinternal rim of the wall 13.

The sample holder 9 is provided in its upper part with a valve 16 thatopens automatically when a predetermined internal pressure in the sampleholder 9, which is subjected to the action of heat during decompositionof a sample material contained in it, is exceeded, so that the internalpressure can escape to the exterior and be reduced. This serves toprevent the internal pressure from exceeding a predetermined value andoverloading the sample holder 9 or even causing it to explode. The valvemember of the valve 16 is the lid 14 itself.

The valve seat of the valve 16 is formed on the upper rim of thecylindrical wall 13 of the housing and comprises the upper concentric,radially annular rim surface 18 of the cylindrical wall 13 and the rimflange 13a thereof. The lid 14 has, on the outer periphery of itsunderside, a correspondingly shaped radial, annular sealing surface 19.The diameter of the disc-shaped lid 14 corresponds approximately to, andmay be slightly larger than, the external diameter of the housing 11.The lid 14 also has on its underside a centering projection 14a whichfits into the housing 11. The horizontal lower surface of thisprojection is designated 21. Concentrically on the top of the lid 14there is a circular recess 22 with a flat, radial bottom surface 23. Forsafety the lid 14 is provided in the middle with a rated break point 24,preferably formed by a weakening of the disc-shaped lid 14, for examplein the form of an annular groove or a central blind hole 25, preferablylocated in the top of the lid 14.

On top of the lid 14 there is a disc-shaped insert 27 which is insertedinto and substantially fills the recess 22 with its radial bottomsurface resting on the bottom surface 23 of the recess 22 and itsperipheral surface preferably likewise in contact with the hollowcylindrical internal wall of the recess 22. The diameter of the circularinsert 27 is such that it projects beyond the inner edge of the rimsurface 18, and may correspond approximately to the external diameter ofthe housing wall 13 or of the rim flange 13a. The insert 27 may howeverhave a larger diameter than the housing 11. On the top of the insert 27and concentric therewith there is a circular recess 28 with a flat,radial bottom surface 29.

Each sample holder 9 is provided with a disc-shaped, elastic thrustpiece 31 that is inserted into the recess 28 on the lid 14 for thefunctional operation of the sample holder 9. Because of its elasticitythe thrust piece allows the valve 16 to be prestressed by a definiteamount. The thrust piece 31 is located outside the sample holder 9, andhence in a cold region around which air can circulate.

The thrust piece 31 may be a disc of solid elastic material or a bodyshaped as a plate spring such as a hollow, in particular dome-shaped,body, and is preferably an annular or closed plate spring In theembodiment shown in FIGS. 2 to 4 the plate-spring-shaped body ispositioned so that it diverges downwards with its outer peripheral rimseated in the recess 28 with clearance or circumferential play therein.

The plate spring 31 is shaped as the frustum of a cone with a radiallyflat head 31a. Preferably the lower rim is also provided with a radialannular foot surface 31b. The tip of the space under the plate spring 31may run to a point or it may be flattened. The flank wall is preferablyof uniform thickness, cf. the thickness d. The plate spring 31 may beprovided with a central hole and thus be annular, or it may be closedcomponent. The plate spring 31 has a spring excursion of up to about 2to 3 mm over which its prestressing force is or remains substantiallythe same.

The sample holder 9 consists of microwave-transparent and chemicallyinert material, in particular plastics material. A suitable material forthe housing 11 and the lid is a heat-resistant plastic, preferably apolyfluoroethylene derivative, that is relatively soft, particularly athigher temperatures (PTFE, Teflon, TFA, PFA). The thrust piece 31consists of a hard or viscoelastic plastic, preferably a HTC (hightemperature compound) plastic such as a polyetherimide (PEI) orpolyether-ether-ketone (PEEK). Alternatively it may consist of anelastomeric material such as, for example, silicone or Viton. The insert27 consists of a hard plastic that provides the insert with highresistance to deformation and bending, particularly at differenttemperatures, and thus with high pressure stability The pressurestability and bending strength of the insert 27 is greater than thepressure stability of the plate spring. The same applies to adisc-shaped thrust piece of solid elastic material.

The pot-shaped housing 11 is preferably surrounded, preferably over itswhole height, by a tubular protective case 35, to which it is firmlyconnected, for example by a force fit or adhesion. The protective casepreferably consists of plastic having high rupture strength such as apolyetherimide (PEI) or polyether-ether-ketone (PEEK), for example ofthe same material as the plate spring 31, as previously described. Theprotective case 35 may be formed with an opening through which thetemperature on the outer surface of the housing 11 can be measured bymeans of a temperature sensor in order to control the rotary drive ofthe carrier 5, the heating energy and/or the heating time of themicrowave furnace.

For each of the sample holders 9 the carrier 5 has a bearing part thatengages over the thrust piece 31 of the respective sample holder 9 whenit is placed or stood in position on or in the carrier 5, so that thesample holder is held between the bearing part and the base surface 36on the carrier 5 under a stress such that when the internal pressure inthe sample holder 9 exceeds a predetermined value the lid 14 liftsagainst the stress and thus opens the valve 16. Preferably the bearingpart, which is indicated generally by 37, is provided, coaxially witheach of the sample holder positions 8, with a stressing device 38 bymeans of which the prestressing of the valve 16, which is predeterminedby the resilience of the elastic thrust piece 31, may be effected.

In the present embodiment the bearing part 37 is a preferably circularupper flange or disc 39 of the carrier 5 that is connected to the lowerplate 6 by a vertical shaft 40, for example integrally or by means ofscrews (not shown). Coaxially above each sample holder position 8 anexternally threaded setting screw 41 is screwed into a correspondingvertical threaded hole in the upper flange or disc 39. The lower end ofthe setting screw 41 projects slightly downwards from the bottom surfaceof the upper flange or disc 39. At its upper end the setting screw 41 isprovided with a tool-engaging element, for example an internal orexternal hexagon.

In the operating position the setting screw 41 is located coaxiallyabove the lid 14 and the thrust piece 31. By tightening the settingscrew 41 the thrust piece 31 and the lid 14 are forced downwards againstthe housing 11 to produce a definite pressure. The arrangement may besuch that in the tightened position the setting screw 41 comes upagainst a stop: for example, the screw may have a head that meets theupper flange or disc 39.

To set particular desired prestressing forces or opening pressures forthe valve 16 a corresponding number of thrust pieces 31 may be providedfor each sample holder 9, these thrust pieces having an elasticity orresilience that is correspondingly different and adapted to therespective desired opening pressure.

In order to give each sample holder 9 a definite position in the carrier5 a recess 45 is provided at each holder position 8. This recess ispreferably open radially outwards so that the sample holder 9 concernedcan conveniently be pushed in from the side. At least in its inner partthe recess 45 is so shaped and/or adapted to the cross-sectional shapeof the sample holder 9 that this is centered in the correct position.

The carrier 5 can be rotated about the axis of rotation 7 by a drive(not shown) so that the sample holders 9 are uniformly heated.

If it is desired to prevent contamination of the heating chamber 2 ofthe heating appliance 1 the carrier 5 may be provided with a protectivehousing 46, indicated in outline, that can preferably be slipped overthe carrier 5 from above. Preferably the peripheral wall of the lowerplate 6 is narrowed by an upwardly-facing step 47 to form a rim on whichthe protective housing 46, which may be bell-shaped, can stand. Theprotective housing 46 preferably consists of transparent material suchas glass or plastic, so that the sample holder 9 and if desired also itsreaction on heating is visible from outside.

For safety the carrier 5 and the plate 6 and/or flange or disc 39 mayalso be arranged so that the respective bearing part and/or at least oneof the two plate 6 and flange or disc 39 can bend slightly outwardselastically or yield elastically under the opening pressure exertedupwards by the respective sample holder 9 when in use. In this way theseparts perform a secondary opening function for the valve 16, to ensurethat the valve 16 opens if the internal pressure is exceeded by morethan the normal amount.

The carrier 5 and/or the plates 6, and flange or disc 39 also consist ofplastic material. The additional opening function just described isassisted if the heat of reaction is transmitted to the plate 6 andflange or disc 39 and the plastic material thereof is softened and itsstrength is reduced, so that one or other of the plate 6 or the flangeor disc 39 can bend outwards. Preferably the heat-resistance of thisplastic is less than that of the protective case 35 and/or of the thrustpiece 31, so that this plastic melts sooner. To avoid prematuredeformation of the plate 6, the bottom 12 of the housing 11 is madethicker than its wall 13 for the purpose of thermal insulation.

The setting screw 41 preferably likewise consists of plastic, namely ofa hard plastic, to prevent the plate spring pressing into the settingscrew 41.

When the pressure in the sample holder increases as the sample materialis decomposed, the pressure on the lid at first increases, since theplastic of the container expands. When the internal pressure produces aforce acting on the lid 14 from the inside that exceeds the resilienceof the thrust piece 31, the opening pressure is reached and the lid 14lifts up slightly while remaining parallel and the internal pressure isreleased between the surfaces 18, 19 of the sample holder 9.

In the embodiment shown in FIG. 3, in which the same reference numeralsare used for similar parts, an inner container 51 consisting preferablyof quartz is provided as an additional component that can be insertedloosely into the housing 11 from above and has a flange projection 52 onits rim with which in its inserted position it lies on the rim surface18 of the housing 11. In this embodiment the lid 14 has an annulargroove 53 to receive the rim or flange projection 52 of the innercontainer 51, so that the centering is maintained by means of acentering projection of the lid 14 that fits into the inner container 51and the sealing surface 19 of the lid 14 still rests on the rim surface18 of the housing 11 to ensure sealing of the sample holder 9. Here thebottom surface 53a of the annular groove functions as a sealing surfacefor the inner container by bearing tightly thereon. The bottom of theinner container 51 is preferably hemispherical so as to be free frombending forces arising from the internal pressure. The inner container51 can therefore be made of fragile material, in particular quartz.Quartz is particularly suitable since it is chemically inert andtransparent to microwaves, and also provides significant heatinsulation.

The hollow cylindrical inner container 51 has a smaller internaldiameter than the housing 11, with the result that the effectivehorizontal internal cross-sectional area is smaller. Consequently forthe same opening pressure there is a smaller opening force acting on thelid 14, and the valve 16 and the lid 14 will only open at a higherinternal pressure and consequently at a higher reaction temperature. Byusing the inner container 51 the user can thus obtain a correspondinglyhigher reaction pressure and higher reaction temperature with the use ofthe same thrust piece 31.

In order to increase the load limit, particularly when using an innercontainer 51 of fragile material such as quartz, it is advantageous tointroduce a liquid, in particular water, into the space 54 between thebottom of the inner container 51 and the bottom of the housing 11. Thewater is heated by the microwave radiation and by the heat given off bythe inner container 51 and accordingly generates a vapour pressure thatcan amount to as much as about 60 bar. In the present embodiment theplate spring forming the thrust piece 31 produces a closure force ofabout 10 000 N (1000 kPa), so that the valve 16 opens at an internalpressure of about 110 bar. If however the user wishes to operate with alower internal pressure of e.g. 60 bar and a correspondingly lowertemperature, he can use the sample holder 9 without its inner container51.

In the embodiment shown in FIG. 4, in which the same reference numeralsare used for similar parts, the joint between the rim surface 18 of thehousing 11 and the sealing surface 19 of the lid 14 is covered radiallyoutside by a guard ring 56 which surrounds the continuous peripheraljoint closely or with a slight clearance. A radial opening 57 isprovided through the guard ring 56 with its middle axis approximatelylevel with the joint. Alternatively a plurality of holes 57 may beprovided, distributed around the periphery. In the embodiment shown inFIG. 4 the guard ring 56 is a separate component that can be pushed overthe lid 14 and over the rim flange 13a from above with clearance, sothat the guard ring 56 rests on the upper rim surface of the protectivecase 35, the diameter of which is rather greater than that of the lid 14and the rim flange 13a, which in the present case are the same size. Theguard ring 56 likewise consists of a microwave-transparent andchemically inert plastic, preferably of the same plastic as the lid 14and the housing 11 (PTFE derivative).

Another possibility is to provide the guard ring 56 with an internalflange on its upper rim, with which it can rest on the upper side of thelid 14. With this arrangement the guard ring 56 does not have to rest onthe protective case 35, which can then be made smaller or thinner. It isalso within the scope of the invention to form the guard ring 56integrally on the outer rim of the rim flange 13a of the housing 11 oron the upper rim of the protective case 35, projecting upwards fromthese parts. It is also possible to form the guard ring 56 on theperiphery of the lid 14 so that it projects downwards therefrom.

What is claimed is:
 1. A sample holder for decomposition or analysis ofsample material in a heating appliance, said sample holder comprising areceiving part having an upper peripheral rim which defines a fillingopening for receiving sample material and which also forms a valve seat,a lid which has a resistance to deformation and bending for closing thereceiving part at the filling opening, a moveable valve member formed onone side of said lid, said valve member being configured to close onsaid valve seat and an elastically yielding thrust piece formed by aplate spring with a dome shaped body having flanks and which diverges toan outer peripheral rim, said thrust piece being positioned to presselastically against said lid on the side thereof opposite said valveseat.
 2. A sample holder according to claim 1 wherein the valve seat issituated at the upper peripheral rim of said filling opening, andwherein the valve member is formed by the lid and rests loosely on theupper peripheral rim.
 3. A sample holder according to claim 1, whereinthe valve member is a flat plate.
 4. A sample holder according to claim1, wherein the thrust piece has a first dimensional stability andwherein an insert having a second dimensional stability is providedbetween the valve member and the thrust piece, said second dimensionalstability being greater than said first dimensional stability.
 5. Asample holder according to claim 4, wherein said valve member has athird dimensional stability and wherein said third dimensional stabilityis less than said second dimensional stability.
 6. A sample holderaccording to claim 1 wherein the dome shaped body of the plate springdiverges towards the lid.
 7. A sample holder according to claim 1wherein the thrust piece has a center and a radial head surface at saidcenter and a radial annular foot surface at said outer peripheral rim.8. A sample holder according to claim 4, wherein the thrust piece andthe insert consist of microwave-transparent and chemically inertmaterial.
 9. A sample holder according to claim 1, wherein a rated breakpoint is provided in the valve member.
 10. A sample holder according toclaim 1, wherein said receiving part includes an inner container ofmicrowave-transparent and chemically inert material and having a rimextending upwards as far as the lid.
 11. A sample holder according toclaim 10, wherein said rim of said inner container has anoutwardly-projecting flange which rests on a shoulder on said rim of thereceiving part, and wherein an annular groove to receive the projectingflange is formed in said one side of the lid.
 12. A sample holderaccording to claim 10 wherein said inner container has a hemisphericalbottom.
 13. A sample holder according to claim 1, wherein valve surfacesbetween the receiving part and the lid are formed by a radiallyextending rim surface of the receiving part and an opposing annularsurface on said one side of the lid, and a guard ring surrounding saidsurfaces, said guard ring being of microwave transparent and chemicallyinert material.
 14. A sample holder according to claim 13, wherein theguard ring is provided with means defining at least one opening at thelevel of the valve surfaces.
 15. A sample holder according to claim 1,wherein said plate spring is made of a microwave-transparent andchemically resistant plastic material.
 16. A sample holder according toclaim 1, wherein said valve seat is situated at an edge portion of saidfilling opening and wherein said lid is formed with a centeringprojection which fits inside the filling opening.
 17. A sample holderaccording to claim 1, wherein said valve seat is located at the rim ofsaid receiving part and wherein said valve member is formed as part ofsaid lid.